For a variety of reasons, it is often medically desirable to replace a section of a blood vessel, either a vein or an artery, with a prosthesis rather than using a viable tissue graft. A difficulty frequently encountered in the replacement of a natural vessel section with a prosthesis is that, despite successful surgical implantation, the prosthesis occludes and thereby fails in its function. This problem is particularly troublesome with what can be described as small vessel prostheses, particularly those intended to replace or repair small arterial sections. The resulting blockage in the artery (lumen occlusion) usually occurs at the sites of the anastomoses and, if the prosthetic material is adequately blood compatible, is primarily due to tissue reactions such as endothelial, smooth muscle cell, or fibroblast proliferation, rather than clot formation. There is evidence to suggest that mismatch at the suture line between the elastic properties of the host vessel and the prosthesis is a primary contributor to mechanisms underlying these reactions.
All vessel materials manifest several important viscoelastic properties, the determination of the significance of these properties is dependent upon the application, loading conditions, loading frequencies and other variables. The nonlinear elastic modulus of most biomaterials dominates their mechanical behavior in a vessel application; the variation in this property between natural and prosthetic materials can span orders of magnitude. If a vessel is sutured to a prosthesis with an identical unloaded geometry, the relatively smaller load bearing surface of the sutures tends to impart significant stress concentration at the interface, particularly if the prosthesis has a different elastic modulus. For example, a small artery with a lumen diameter on the order of 2 mm (0.08 inches), developing a peak strain on the order of 0.1, can develop peak azimuthal wall stresses on the order to 10.sup.5 -10.sup.6 dyne/cm.sup.2 (1.45-14.5 lb/in.sup.2). If a natural vessel is sutured to a prosthesis with an elastic modulus one order of magnitude greater, the resulting stress at a suture can easily achieve a value two orders of magnitude greater than the normal peak azimuthal stress. This increased stress burden brought about by the mismatch can cause severe tissue reactions which may lead to the failure of the implant.
As might be expected, reactions common to viable tissue under stress, e.g., fibroblast and muscle cell proliferation or hypertrophy, collagen synthesis, venous graft "arterialization", etc., can occur in the vessel. For example, research has demonstrated that smooth muscle cell proliferation occurs as a direct consequence of cyclic tensile stress. See, Leung, et al. "A New In Vitro System for Studying Cell Response to Mechanical Stimulation", Experiment Cell Research, Vol. 109, pp 285-89, (1977), incorporated herein by reference.
If all other factors remain constant, as lumen diameters increase, wall stress increases in direct proportion to the lumen radius. However, since the cross-sectional area increases in proportion to the square of the radius, the occlusive problem is progressively diminished with increasing lumen diameter, despite similar reactions at the site of the anastomosis. Therefore, although stress reduction is important in all vessel prosthetics, the problem is particularly acute in the range of lumen diameters below about four millimeters. See, Proceedings, Workshop on Blood, Transport Phenomena, and Surfaces, National Institutes of Health Publication No. 86-2726 (1986) (hereinafter "NIH Workshop"), incorporated herein by reference.
Previous attempts to solve the problem of stress-induced tissue reactions have largely concentrated on the material surface properties, as well as the mismatch of elastic properties between the vessel and the prosthesis--in other words a search for a "perfect material" to be used as a prosthesis. Progress has been made by selecting materials and constructing prostheses for particular site-specific applications. Other research continues. In particular, studies of material properties such as porosity and compliance, as well as studies of the mechanical properties of diseased arteries have been identified as promising areas of research. See, NIH Workshop at 77-78.
Although many areas of research hold promise, there is currently a need within the medical profession to have the capability to implant reliable prostheses in patients with diseased or damaged vessels. As reflected by the current state of the art, there exists at this time no general solution to this problem since no material has been found to have the requisite surface and mechanical properties while being biomedically acceptable as a prosthesis. There is, therefore, a long-felt but unsolved need for a small diameter prosthetic vessel having improved elastic properties to eliminate or substantially reduce the stress imparted at the anastomosis.